In my experience healing the normals in DS so far; I would say that I agree with most of ur evaluations of difficulty, however I disagree with the following two:
-Hagara: This I can say straight up is dead easy with two (might even be possible to solo heal it).
-Ship: I've only ever done with two healers since its nice to be able to dps the adds down faster.
The last two are where it becomes really interesting. I can't really give an informed opinion on them since I've only ever three healered them, but, my guess, from the healing I have done on them is that Madness would probably be easier to two heal than spine. I say that because Madness requires a lot less healing for a lot less of them time, right up until he face plants in between those two platforms, and Spine is a lot more sustained and might stress your mana (don't take my word for it though since I haven't two healed them).
I think you'll run into problems on HC though. I'm only 1/8HC, and can say from experience that Morchock HC will be extremely tough with two healers. I know Yorsahj is two healed to miss the enrage, but my initial guess would be that you want to make sure that you can, if necessary, take three healers to the later fights.
Over all I would advise to keep looking for a third just because only having two quite clearly causes some issues in the flexibility of your raid team.
Hope this was helpful